Embryonic stem cells (ESCs) are cells derived from blastocysts obtained from in vitro fertilization that have the potential to self-renew and to differentiate into any mature cell type of a mammalian, e.g. human, body (this property is know as “pluripotency”). Under specific tissue culture conditions, ESCs can be maintained undifferentiated for prolonged periods of time without loosing their pluripotent characteristics. Because of these properties, ESCs have aroused enormous interest in regenerative medicine. Potentially, tissues derived from ESCs could be used for clinical treatment of humans (either acute conditions or genetic and degenerative diseases). ESCs could also be used for drug screening, for example to study tissue-specific susceptibility to treatments, and to model genetic diseases in vitro. However, severe ethical and practical (the risk of immune rejection) limitations have seriously hampered application of ESCs in the clinic and, in many countries, also the research.
The recent discovery that somatic cells can be transformed into iPSCs by means of exogenous factors (this method has also been termed “nuclear reprogramming by exogenous factors” (Takahashi and Yamanaka, Cell 2006; 126:663-76) has the potential to change the current perception of personalized medicine and can also provide valuable in vitro models of human diseases (Yamanaka and Blau, 2010; Nature 465, 704-712; Lian et al., 2010; Thromb Haemost 104, 39-44).
Remarkably, iPSCs are similar to ESCs and have the potential to be used in patient-specific treatments, thus avoiding the risk of immune rejection. Because of these characteristics, iPSCs have received great attention worldwide. The generation of iPSCs requires the collection of tissue from the donor, the expansion of the donor cells in vitro, and the exposure of the cells to a cocktail of exogenous factors that are provided to the cell as purified proteins, protein extracts, RNA molecules, non-integrating plasmids, viruses (e.g. retroviruses, lentiviruses, adenoviruses, Sendai viruses), with or without chemical cocktails and with variations in the cell culture conditions.
Application of these factors has the effect that colonies with an ESC-like morphology progressively emerge. These colonies represent iPSC colonies that can be picked and subsequently expanded and characterized to verify that their behavior is similar to ESCs.
So far, human iPSCs have been generated using donor cells from skin (fibroblasts and keratinocytes), amniotic fluid, extra-embryonic tissues (placenta and umbilical cord; (Cai et al., 2010; J Biol Chem 285, 11227-11234) cord blood, periosteal membrane, dental tissue, adipose tissue, neural stem cells, hepatocytes, amnion-derived mesenchymal stem cells and peripheral blood cells (Ye and Cheng, 2010; Regen Med 5, 521-530; Cai et al., 2010; J Biol Chem 285, 11227-11234). Reprogramming of cells from these tissues has been achieved with varied frequencies, indicating that the cell of origin (“donor cell”) matters. Due to the heterogeneity of the donor cells currently used for iPSC generation, it is difficult to set standards for performing comparative tests with iPSCs and ESCs and to draw meaningful conclusions from the test results.
The ideal donor cell type for generating iPSCs should be easily accessible, easily reprogrammable, and universal (any age, sex, ethnic group, and body condition). Nowadays, dermal fibroblasts are among the most frequently used cell sources for reprogramming, but they require not only an uncomfortable biopsy that needs to be performed in an aseptic environment by a specialist, but also prolonged expansion prior to use. They also have risk of somatic cell mutations due to exposure to light and radiation, and the procedure is contraindicated in severe skin diseases or burns. Recently, three research groups achieved reprogramming of peripheral blood cells without need of CD34+ cell mobilization was reported (Loh et al., 2010; Cell Stem Cell 7, 15-19; Seki et al., 2010; Cell Stem Cell 7, 11-14; Staerk et al., 2010; Cell Stem Cell 7, 20-24). Brown et al., 2010, PloS ONE, 2010, 5, 6, 1-9, describe the generation of T lymphocyte-derived iPSCs from peripheral blood. Since these procedures are minimally invasive, require small blood quantity and do not need prolonged cell culture, they represent a significant advance inspite of the fact that their efficiency is very low. However, the main donor cells used according to these reports are mature T-cells that bear specific T-cell receptor rearrangements, which is undesirable for certain potential clinical applications. Also, T-cells do not carry the complete genetic information that is required for unlimited differentiation into any cell type.
Besides, in rare cases, receiving/donating blood is not exempt from ethical concerns, for example because of religious beliefs, and may not be feasible in patients with infectious diseases, blood diseases, or immunodepression. In the latter context, conditions are to be considered that affect coagulation (e.g. hemophilia), leukemia and genetic or acquired (e.g. cancer and AIDS) immunodepression.
In search for reprogramming of new tissues, iPSCs have also been produced from mouse meningeal membrane (Qin et al., 2008; J Biol Chem; 283, 33730-33735) and mammary epithelial cells (Li et al., 2010; Cell Stem Cell 7, 51-63), and in humans from periosteum and adipose stem cells (Esteban et al., 2010; Cell Stem Cell 6, 71-79), umbilical cord matrix and placenta (Cai et al., 2010; J Biol Chem 285, 11227-11234).